Improved hydrocarbon compositions are needed to help meet the growing demand for middle distillate products, such as aviation turbine fuels, for example, JP-8 and diesel fuel. Diesel fuel generally provides a higher energy efficiency in compression ignition engines than automotive gasoline provides in spark combustion engines, and has a higher rate of demand growth than automotive gasoline, especially outside the U.S. Further, improved fuel compositions are needed to meet the stringent quality specifications for aviation fuel and the ever tightening quality specifications for diesel fuel as established by industry requirements and governmental regulations.
One known route for producing hydrocarbon compositions useful as fuels is the oligomerization of olefins over various molecular sieve catalysts. Exemplary patents relating to olefin oligomerization include U.S. Pat. Nos. 4,444,988; 4,497,968; 4,482,772; 4,720,600 and 4,879,428. In these disclosures, feedstock olefins are mixed with an olefinic recycle material and contacted with a zeolite, particularly in a series of fixed bed reactors. The oligomerized reaction product is then separated to provide a distillate stream, and typically a gasoline stream, and any number of olefinic recycle streams.
However, in these known oligomerization processes, the focus is on producing relatively heavy distillate products, and even lube base stocks. To enable the production of relatively heavy materials, the processes employ, either directly or indirectly, a relatively large amount of olefinic recycle containing significant quantities of C10+ material. The relatively large recycle rate provides control over the exotherm of the oligomerization reaction in the preferred fixed bed, adiabatic reactor system, while the relatively heavy recycle composition enables the growth of heavier oligomers and thus higher molecular weight and denser distillate product. A high rate of recycle requires much larger equipment to handle the increased volumetric flow rate, and uses more separation/fractionation energy, and hence more and larger associated energy conservation elements. Further, a high molecular weight oligomer product requires very high temperatures for the fractionation tower bottoms streams that may eliminate the use of simple steam reboilers and require more expensive and complicated fired heaters.
The recycle streams in conventional olefin oligomerization processes are produced in a variety of rather complicated fashions with numerous unit operations, typically including some sort of single stage flash drum providing a very crude separation of reactor product as a means of providing some of the relatively heavy components, followed by various fractionation schemes which may or may not provide sharper separations, and again often providing heavy components as recycle. Further, following the art of olefin oligomerization as it progressed through the 1980s and 1990s, one is directed to introducing various components with the feed to the reaction zone to prolong catalyst cycle life. These additional components further complicate subsequent separation efforts, for example, the addition of hydrogen as disclosed in U.S. Pat. No. 4,544,792 (and in U.S. Pat. No. 4,879,428 noted above, among others). The dense distillate product is generally characterized by a relatively high specific gravity (in excess of 0.775) and a high viscosity, in part due to the composition comprising relatively high levels of aromatics and naphthenes.
Very few references discuss both the merits and methods of producing lighter distillate products, typified by such as jet fuel, kerosene and No. 1 Diesel, via the oligomerization of C3 to C8 olefins. Jet/kero is generally overlooked as a particularly useful middle distillate product, inasmuch as the volume consumed in the marketplace is considerably smaller than its heavier cousins, No. 2 Diesel and No. 4 Diesel (fuel oil). However, jet/kero is a high volume commercial product in its own right, and is also typically suitable as a particular light grade of diesel, called No. 1 Diesel, that is especially useful in colder climates given its tendency to remain liquid and sustain volatility at much lower temperatures. In addition, jet/kero type streams are often blended in with other stocks to produce No. 2 Diesel, both to modify the diesel fuel characteristics, and to allow introduction of otherwise less valuable blendstocks into the final higher value product.
U.S. Pat. No. 4,720,600 discloses an oligomerization process for converting lower olefins to distillate hydrocarbons, especially useful as high quality jet or diesel fuels, wherein an olefinic feedstock is reacted over a shape selective acid zeolite, such as ZSM-5, to oligomerize feedstock olefins and further convert recycled hydrocarbons. The reactor effluent is fractionated to recover a light-middle distillate range product stream and to obtain light and heavy hydrocarbon streams for recycle. The middle distillate product has a boiling range of about 165° C. to 290° C. and contains substantially linear C9 to C16 mono-olefinic hydrocarbons, whereas the major portion of the C6 to C8 hydrocarbon components are contained in the lower boiling recycle stream, and the major portion (e.g., 50 wt % to more than 90 wt %) of the C16+ hydrocarbon components are contained in the heavy recycle fraction.
According to the present invention, it has now been found that by controlling the ratio of components in the recycle stream, a simpler and more efficient process results to produce fuel products, particularly light distillate products and blendstocks, such as jet fuel, kerosene, and diesel fuel.